Rod-like, extended-chain, aromatic-heterocyclic polymers have received considerable interest in both academic and industrial laboratories over the past two decades. These polymers generally fall into two classes, those that are modified by temperature changes, i.e., thermotropic liquid crystalline polymers and those that are modified in solution form, i.e., lyotropic liquid crystalline polymers. These polymers are hereinafter sometimes referred to as thermotropic and lyotropic LCPs. For a short hand expression covering both types of polymers, the term "ordered polymers: will also sometimes be used herein.
As used herein, "ordered polymers," "extended-chain aromatic-heterocyclic ordered polymers," thermotropic," and "lyotropic" liquid crystalline polymers (homopolymers, copolymers, and the like) all refer to one or more of the known classes of polymers having a fixed molecular orientation in space, i.e., linear, circular, star shaped, or the like. Such polymers are disclosed in the following patents:
U.S. Pat. No. 4,423,202 to Choe, discloses a process for the production of para-ordered, aromatic heterocyclic polymers having an average molecular weight in the range of from about 10,000 to 30,000.
U.S. Pat. No. 4,377,546 to Helminiak, discloses a process for the preparation of composite films prepared from para-ordered, rod-like, aromatic, heterocyclic polymers embedded in an amorphous heterocyclic system.
U.S. Pat. Nos. 4,323,493 and 4,321,357 to Keske et al., disclose melt prepared, ordered, linear, crystalline injection moldable polymers containing aliphatic, cycloaliphatic and araliphatic moieties.
U.S. Pat. No. 4,229,566 to Evers et al., describes para-ordered aromatic heterocyclic polymers characterized by the presence of diphenoxybenzene "swivel" sections in the polymer chain.
U.S. Pat. No. 4,207,407 to Helminiak et al. discloses composite films prepared from a para-ordered, rod-like aromatic heterocyclic polymer admixed with a flexible, coil-like amorphous heterocyclic polymer.
U.S. Pat. No. 4,051,108 to Helminiak et al., discloses a process for the preparation of films and coatings from para-ordered aromatic heterocyclic polymers.
Ordered polymer solutions in polyphosphoric acids (including PBZ compositions) useful as a dope in the production of polymeric fibers and films are described in U.S. Pat. Nos. 4,533,692, 4,533,693, and 4,533,724 (to Wolfe et al.).
The disclosures of each of the above described patents are incorporated herein by reference.
Thermotropic and lyotropic LCPs are of great interest, because they exhibit a partially ordered state that is intermediate between a three dimensional ordered crystalline state and the disordered or isotropic fluid state. As a consequence of the molecular ordering, LCPs are anisotropic (i.e., their properties are a function of molecular direction (R. A. Weiss and C. K. Ober, "Liquid-Crystalline Polymers," ACS Symposium Series 435, (1990)).
Structurally, most commercial LCPs consist of rigid mesogenic monomer units connected with either flexible spacers or "kink structures" to make them tractable and processable. The high degree of molecular order that can be achieved with the LCP molecules allows this material to attain a very tight packing density similar to a log jam in a river. LCPs derive their outstanding properties from this tightly packed rigid-rod formation which at a macroscopic level results in a structure that is self-reinforced through the strong interaction of electron deficient and electron rich benezene rings. However, the rigid-rod LCP molecule is highly anisotropic. The axial CTE of the LCP molecule is typically -5 to -10 ppm/.degree.C. while the radial CTE is highly positive (.apprxeq.+70 ppm/.degree.C.). This characteristic in the past has been one of the major reasons why LCP substrates have not been successfully used in electronic packaging applications.
Standard processing of the LCPs through slit dies typically results in a uniaxial film (all molecules aligned in one direction) with a highly negative CTE in the extrusion direction and a highly positive CTE in the transverse direction. This characteristic is undoubtedly unacceptable for, e.g., a chip-supporting substrate that will be exposed to thermal cycles. The resulting in-plane CTE mismatch between the substrate and the chip carrier will result in solder joint failures at the substrate/chip interface. In addition, uniaxial LCP films have virtually no transverse strength and uniaxial tubes have virtually no circumferential strength, and when placed under slight stresses can readily split in the fibril direction.
Novel LCP extrusion technologies disclosed, e.g., in U.S. Pat. Nos. 4,871,595; 4,939,235; 4,963,428; and 4,966,807; as well as in copending Ser. No. PCT/U.S. 90/03394, filed Jun. 18, 1990, (hereinafter referred to as "Extrusion Processes") enable the production of film-based and tubular components of thermotropic and lyotropic LCPs that have highly controlled biaxial orientation, resulting in films and tubes that have property balances that are much more useful from a practical standpoint than ordinary uniaxially referred to above are incorporated herein by reference. Films having a balanced biaxial orientation are particularly suitable for electronic packaging applications. Tubes with biaxial orientation are particularly suitable for structural applications. The biaxial orientation of such films and tubes are the result of a two stage orientation process, one of which occurs in a counter rotating-die, followed by post treatment to optimize the film property balance as disclosed in the Extrusion Processes.
Film-based and tubular components produced in accordance with the Extrusion Processes provide significant technological advances over standard LCP processing techniques by minimizing the problems associated with poor transverse and circumferential properties and providing techniques for tailoring various properties such as CTE, tensile strength and modulus (Blizard et al., American Chemical Society Preprints, Vol. 32, No. 2, 1991). This is accomplished by aligning the LCP molecules along two principal axes within a single ply.
The angle between the two principal axes on either side of, e.g., an LCP film, and the extrusion direction are normally balanced and identified by a .+-..theta. nomenclature. The molecular orientation through the thickness of the film gradually changes from +.theta. to -.theta.. FIG. 1 illustrates the difference in molecular orientation between uniaxial oriented LCP film (FIG. 1A) and controlled biaxially oriented LCP film (FIG. 1B) produced in accordance with the Extrusion Processes. The arrows in FIGS. 1A and 1B indicate machine direction. FIGS. 2 and 3, scanning electron microscope (SEM) photographs of a thermotropic LCP film having controlled in-plane biaxial orientation and prepared in accordance with the Extrusion Processes which was frozen in liquid nitrogen and broken to reveal the internal fibrillar structure, also illustrate how the molecular orientation of the film gradually changes through its thickness. The lines indicate fibril orientation.
A combination of shearing and stretching during the counter-rotating dies used in the Extrusion Processes orients the LCP molecules/fibrils. A schematic of one such counter-rotating die is shown in FIG. 4. This schematic shows the following components of the counter-rotating die: drive 1, bearings 2, distribution block 3, outer rotor 4, inner rotor 5, film 6, outer and inner rotors 7, circumferential shear pattern 8, filter/strainer 9, and LCP from pump 10. The biaxial angle that the LCP fibrils make with the longitudinal axis of the tubular extrudate or film can be readily varied from .+-.5 to .+-.70 degrees. The rotation of the counter-rotating mandrels creates transverse shear flows that are superimposed on the axial shear developed as the polymer melt is extruded through the die. This operation presets the biaxial orientation. Subsequent post-die draw and blow, as shown in the counter-rotating die extrusion process schematic presented in FIG. 5, is used to further adjust and enhance the biaxial orientation. FIG. 5 shows the following components: desiccant/drier heater 20, hot dry air in 21, metering auger 22, gear pump 23, 200 mesh coarse filter 24, one inch extruder 24:1 L/d, 6:1 compression ratio 25, counter-rotating die 26, blow up ratio 27, draw rate 28, blown film 29, convergence rollers 30, and pinch rollers 31.
When LCPs are processed into balanced biaxial films, i.e., about .+-.45 degrees, physical properties such as CTE, tensile strength and modulus are about equal in any in-plane direction. This control of CTE through control of molecular orientation is graphically illustrated in FIG. 6 where the machine (extrusion) direction and transverse direction CTEs for 3 mil thick balanced biaxial film of a particular thermotropic LCP are plotted as a function of biaxial orientation. As the biaxial orientation approaches .+-.45 degrees the machine and transverse CTEs converge to +3 to +4 ppm/.degree.C. Other LCPs processed into .+-.45 degree biaxial films have yielded CTEs ranging from -10 to +4 ppm/.degree.C.
Electronic packaging has undergone major technical advances since the 1970s due to the development of large scale integrated chip technology (LSI), very large scale integrated chip technology (VLSI), and very high speed integrated circuit technology (VHSIC). The introduction of hermetic leadless ceramic chip carriers (LCCCs) as a replacement for bulky dual in-line packages (DIPs) and the development of surface mount technology processes have also heightened the need for advanced interconnection substrate materials. Traditional printed wire board (PWB) materials such as fiber reinforced epoxies and polyimides, and alumina-beryllia ceramic cannot take full advantage of the increased input/output (I/O) density, improved reliability and lower cost offered by today's advanced electronic packaging technologies.
For example, the use of LCCGs in surface mount technology applications results in a coefficient of thermal expansion (CTE) mismatch between the ceramic carriers (CTE=6.4 ppm/.degree.C.) and the epoxy-glass or polyimide-glass PWB substrate (11 to 16 ppm/.degree.C.). Large multichip modules (MCMs) with a CTE of 3 to 7 ppm/.degree.C. also have a similar substrate/module CTE mismatch problem. This mismatch in thermal expansion reduces the surface mount component solder joint reliability, especially in applications with frequent or extreme temperature cycles. This failure mechanism is a critical concern, especially in military and space applications where improper operation can have disastrous effects or can result in defects in expensive systems which cannot be repaired.
A variety of techniques are presently used to resolve the PWB/LCCC CTE mismatch problem. Each of these approaches, however, has certain drawbacks. For example, ceramic substrates have the required CTE, but are very brittle and expensive. Their vulnerability to chipping and cracking reduces their reliability. They are also difficult and costly to repair. Furthermore, their high dielectric constant (9.4) also restricts their use in high density VHSIC applications.
New fiber-reinforced composites using Kevlar, graphite or quartz fiber are also being developed to match the LCCC CTE, but these materials also have drawbacks (See, e.g., Belke Jr., et al. Tailorable Coefficient of Thermal Expansion Multilayer PWBs for High Density Leadless Perimeter and Grid Array Packages, Final Report, AFWAL-TR-87-4152, prepared for Materials Lab, Wright Patterson AFB, March 1988.). The thickness of the fiber weave is at least twice the diameter of the fiber. State of the art fabric weave thickness is limited to approximately 0.005 in. Add the thickness of the resin and it becomes difficult to produce thin layer composites. The surface finish is also likely to be affected by the anisotropic weave geometry in very thin sections. Differential thermal expansion between the fiber and matrix promotes microcracking. The discontinuous nature of the weave can also cause nonuniform expansion. The use of papers, matrix and platelets may reduce these problems, but still produce a multicomponent reinforced material rather than a homogeneous substrate (See, e.g., J. Diekman and M. E. Mirhej, Non-Woven Aramid Papers--A New PWB Reinforcement Technology, IEPS Proceedings, Marlborough, Mass, Sept. 1990.). High dielectric constant and costly manufacturing are additional problems associated with these new composites.
Matched CTE substrates may also be made by using high stiffness metal core layers with very low CTE (See, e.g., R. C. Daigle et al., Engineering Printed Wire Boards for Enhanced Surface Mount Reliability, IEPS Proceedings, Marlborough, Mass, Sept. 1990.). These internal metal planes constrain the overall PWB expansion. The most commonly used core material is copper-clad invar (Cu-In-Cu), followed by copper-clad molybdenum and Alloy 42. These metal cores, however, are relatively heavy and require surface and via insulation to prevent circuit shorts.
Recently, more advanced substrates using fluorocarbon based materials have been introduced, particularly for military electronics packaging. These fluorocarbon substrates have an in-plane CTE which matches the metallization CTE, have a low dielectric constant and can be fabricated into relatively thin substrates. These materials, however, are difficult to process, require special processing equipment and because they are compliant, have a low modulus which can lead to vibration induced failures.
Clearly, there is a need for an improved electronic substrate that can improve PWB reliability. Ideally this substrate should have a self-reinforcing structure, low dielectric constant (&lt;3.0), low dissipation factor (&lt;0.05), low moisture absorption (&lt;0.5%), in-plane (x-y plane) CTE of 6 to 7 ppm/.degree.C. for LCCCs and 3 to 7 ppm/.degree.C. for multichip modules, a medium level modulus (0.3 to 1.5 MSI) and be readily fabricated into thin substrates (&lt;2.0 mil).
Thermotropic and lyotropic LCPs are well-suited for advanced PWBs because they possess a low dielectric constant, about 2.8 and 2.6 respectively, and can be fabricated into very thin substrates, 1 to 2 mil and 0.1 to 0.2 mil respectively. These LCPs also have low moisture absorption (&lt;0.1 percent) and excellent barrier properties. Furthermore, thermotropic LCPs have medium level modulus (1.0 MSI) and can be fabricated into MLBs using standard MLB equipment and techniques with minimal retooling.
Improved electronic substrates comprising biaxially oriented thermotropic and lyotropic liquid crystalline polymers having a controlled CTE are disclosed in U.S. Pat. Nos. 4,975,312 and 4,871,595 respectively. However, the in-plane CTE of an as extruded balanced biaxial component is highly dependent on the LCP composition. Accordingly, in some instances it is desirable to further tailor the CTE of such biaxially oriented components, and methods to accomplish this objective are being sought.